You don’t mention what kind of ignition system you are dealing with as the spark plug gap will vary somewhat depending upon the particular form of electronic ignition is being used.  Larger than 0.035” gaps are common on fuel injected engines where the engines are running on the lean side.  Those engines also have some pretty high-powered ignition systems with some having a separate coil for each spark plug or in my case, the V10 in my Ford F250 has ten coils.  In those instances, that larger gap is required to search out and find that leaner mixture.  Just keep in mind that larger spark plug gaps do put an additional load on the ignition system and makes them work harder electrically thus causing them to fail earlier.

But as a general rule, anything with a carburetor will be 0.035” or less for the spark plug gap.  As the compression ratio or cylinder pressure goes up, so too is the need for higher output ignitions.  The work around for higher compression or using some form of boost is to decrease the spark plug gap and running as low as 0.018”-0.020” plug gaps is not uncommon.

Lorena, Texas (South of Waco)

When using Autolight AP46 plugs in a moderate street build. Do you increase the gap or is .035 preferred? I have read that a larger gap provides more spark which is good but will require more energy. I am using a Flame Thower coil and the engine is very well grounded.




.150 Stroked Y-Block:327.25 ci @ >1hp per ci